Centrifugal fan

ABSTRACT

A centrifugal fan includes an impeller arranged to rotate about a rotation axis, a motor arranged to rotate the impeller, and a case arranged to accommodate the impeller and the motor. The case includes an upper case, a lower case, and a joint portion at which the upper and lower cases are fixed to each other. An interior of the case includes a first annular space positioned radially outward of the motor. The first annular space includes a second annular space defined by a portion of a space defined by an annularly continuous collection of imaginary circles each of whose diameter is the shortest distance between an inner circumferential edge of the joint portion and a radially outermost circumferential portion of a motor accommodating portion and each of which touches the inner circumferential edge of the joint portion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a centrifugal fan having high static pressure.

2. Description of the Related Art

Centrifugal fans designed to achieve increased static pressure are known.

US 2009/0301485, for example, describes a blower in which a shroud is arranged at a top portion of an impeller.

JP 2005-510663 describes a turbine including an annular compression chamber which is in communication with a compression chamber.

In the blower described in US 2009/0301485, the shroud arranged at the top portion of the impeller serves to prevent a gas, which has flowed once into a space below the impeller through an air inlet, from flowing backward toward the air inlet. This leads to an increase in ventilation resistance inside an air channel portion, resulting in an increase in static pressure.

However, when there is a demand for an additional increase in the static pressure, it is necessary to further increase the ventilation resistance inside the air channel portion by increasing the diameter of the impeller or by increasing the area of the shroud, which is arranged at an upper end of the impeller. In this case, an increased size of the blower, which is a centrifugal fan, cannot be avoided, and it is difficult to achieve a decrease in the size of the blower. In particular, a decrease in the size of centrifugal fans is demanded in the field of medical appliances, such as respirators and sputum aspirators, since greater portability or the like is demanded in the field.

In the turbine described in JP 2005-510663, air flows into the annular compression chamber (i.e., a space below a plurality of blades), and this causes an increase in ventilation resistance inside the space below the blades. This contributes to preventing the air, which has flowed once into the space below the blades, from flowing backward toward an air inlet, leading to an increase in static pressure of the turbine.

However, the air that has flowed once into the space below the impeller stays in the space temporarily. The air staying in the space is caused to circulate primarily by continuous suction of air through the air inlet. Accordingly, a large circumferential whirl velocity component does not act on the air which has flowed into the space below the impeller. Therefore, circumferential circulation of air (i.e., a flow of air toward an air outlet) is not easily promoted. It is therefore difficult to achieve an improved air volume characteristic. Incidentally, in the field of the medical appliances, such as the respirators and the sputum aspirators, centrifugal fans having high static pressure are desired because of a demand concerning a starting characteristic.

SUMMARY OF THE INVENTION

A centrifugal fan according to a preferred embodiment of the present invention includes an impeller arranged to rotate about a rotation axis; a motor arranged to rotate the impeller; and a case arranged to accommodate the impeller and the motor. The case includes an upper case; a lower case including a motor accommodating portion arranged to cover an outer circumference of the motor; and a joint portion at which the upper and lower cases are fixed to each other. An interior of the case includes a first annular space positioned radially outward of the motor. The first annular space includes a second annular space defined by a portion of a space defined by an annularly continuous collection of imaginary circles each of which including a diameter which is a shortest distance between an inner circumferential edge of the joint portion and a radially outermost circumferential portion of the motor accommodating portion and each of which touches the inner circumferential edge of the joint portion, the portion of the space being defined by a portion of an inner wall of the upper case and a portion of an inner wall of the lower case. A portion of the impeller is arranged in the second annular space.

In the centrifugal fan according to the above preferred embodiment of the present invention, a portion of the impeller is arranged in the second annular space, so that ventilation resistance in the second annular space is increased. This contributes to preventing a gas which has been drawn in through an air inlet from flowing backward toward the air inlet. Moreover, when the gas which has been drawn in through the air inlet circulates in the second annular space, the portion of the impeller which is arranged in the second annular space serves to add a circumferential whirl velocity component to the gas in the second annular space. This leads to efficient circumferential circulation of the gas (i.e., an efficient flow of the gas toward an air outlet). The centrifugal fan is thus able to achieve both an increased static pressure and an improved air volume characteristic while also maintaining a small size.

The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating the structure of a centrifugal fan according to a preferred embodiment of the present invention.

FIG. 2 is a partial cross-sectional view illustrating a first annular space and its vicinity of the centrifugal fan illustrated in FIG. 1 in an enlarged form.

FIG. 3 is a partial cross-sectional view illustrating an example spatial shape of a second annular space according to a preferred embodiment of the present invention.

FIG. 4 is a partial cross-sectional view illustrating the structure of an impeller according to a preferred embodiment of the present invention.

FIG. 5 is a partial cross-sectional view illustrating the structure of an impeller according to a preferred embodiment of the present invention.

FIG. 6 is a partial cross-sectional view illustrating the structure of an impeller according to a preferred embodiment of the present invention.

FIG. 7 is a plan view illustrating the structure of the impeller according to a preferred embodiment of the present invention.

FIG. 8 is a cross-sectional view illustrating the structure of a centrifugal fan according to another preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description of the preferred embodiments, it is assumed that a direction parallel or substantially parallel to a rotation axis is referred to by the term “axial direction”, “axial”, or “axially”, that directions perpendicular or substantially perpendicular to the rotation axis are referred to by the term “radial direction”, “radial”, or “radially”, and that a circumferential direction about the rotation axis is referred to by the term “circumferential direction”, “circumferential”, or “circumferentially”. It is also assumed that an axial direction is a vertical direction, and that a side on which an impeller is arranged with respect to a motor is defined as an upper side. The shape of each member or portion and relative positions of different members or portions will be described based on the above assumptions. Also note that the present invention is not limited to the preferred embodiments described below. It is to be understood by those skilled in the art that variations and modifications can be made appropriately as long as desired effects of the present invention are not, or are substantially not, impaired. Also note that the preferred embodiments described below may be combined with other preferred embodiments of the present invention if so desired.

First Preferred Embodiment

FIG. 1 is a schematic cross-sectional view illustrating the structure of a centrifugal fan 100 according to a preferred embodiment of the present invention.

Referring to FIG. 1, the centrifugal fan 100 according to the present preferred embodiment preferably includes an impeller 10 arranged to rotate about a rotation axis J, a motor 30 arranged to drive and rotate the impeller 10, and a case 19 arranged to accommodate the impeller 10 and the motor 30. An air inlet 40 through which an outside gas is drawn into the centrifugal fan 100 is preferably defined axially above the impeller 10.

The impeller 10 preferably includes a hub 11, a plurality of first blades 12, and a plurality of second blades 13. The hub 11 is substantially in the shape of a circular plate, and is joined to a shaft 31 arranged to rotate about the rotation axis J. The first blades 12 are preferably arranged axially above the hub 11. The second blades 13 are preferably arranged axially below the hub 11.

A peripheral portion of the hub 11 is preferably arranged to project radially outward relative to a peripheral portion of each first blade 12. This arrangement contributes to preventing a gas which has been drawn in through the air inlet 40 due to rotation of the first blades 12 from flowing backward toward the air inlet 40.

Each first blade 12 is arranged to extend axially downward and radially outward away from the rotation axis J. A portion of an inner wall of the case 19 which covers each first blade 12 includes an inner wall surface arranged to extend along an axially upper end portion of each first blade 12. The gas drawn in through the air inlet 40 is caused to flow axially downward and radially outward by the rotation of the first blades 12. That is, a main function of each first blade 12 is to cause the gas drawn in through the air inlet 40 to efficiently flow into the case 19. Moreover, the first blades 12 are arranged to extend radially with the rotation axis J as a center, with each first blade 12 having such an angle that the first blade 12 bends in the same direction as a rotation direction of the impeller 10 as it extends away from the rotation axis J in a plan view. The angle of each first blade 12 at which the first blade 12 bends in the same direction as the rotation direction of the impeller 10 is preferably not constant throughout the radial extent of the first blade 12. Specifically, each first blade 12 is arranged to bend at an increasing angle in the same direction as the rotation direction of the impeller 10 as it extends radially outward away from the rotation axis J. This arrangement enables the rotation of each first blade 12 to easily add a circumferential whirl velocity component to the gas, which primarily includes axially downward and radially outward velocity components. As a result, the gas is more efficiently sent toward an air outlet (not shown, but which is preferably arranged, for example, in a circumferential surface of the case 19).

A main function of each second blade 13 is to send the gas drawn into the case 19 toward the air outlet (not shown).

The motor 30 preferably includes a bearing 32 arranged to support a rotor magnet 33 such that the rotor magnet 33 is rotatable about the rotation axis J, a bearing support portion 35 arranged to support the bearing 32, and a stator 34 supported by the bearing support portion 35. The hub 11, which is fixed to the shaft 31 arranged to rotate about the rotation axis J, includes a hub cylindrical portion 11 a extending axially downward and being substantially cylindrical. The rotor magnet 33 is fixed to an inner circumferential surface of the hub cylindrical portion 11 a. Note that, although the motor 30 illustrated in FIG. 1 is an outer-rotor motor, a motor according to another preferred embodiment of the present invention may be an inner-rotor motor, if so desired.

Regarding the centrifugal fan 100, a torque is produced between the rotor magnet 33 and the stator 34 as a result of a drive current supplied to the stator 34. As a result, the impeller 10 is caused to rotate about the rotation axis J.

The case 19 preferably includes an upper case 20, a lower case 21 including a motor accommodating portion 22 arranged to cover an outer circumference of the motor 30, and a joint portion 23 at which the upper and lower cases 20 and 21 are preferably fixed to each other. The upper case 20 is arranged to extend axially downward and radially outward from a peripheral portion of the air inlet 40. The lower case 21 further includes a radially outer portion fixed to a radially outer portion of the upper case 20 at the joint portion 23, and an intermediate portion arranged to extend axially downward and radially inward in a curve from the joint portion 23, and joined to an upper end portion of the motor accommodating portion 22. Note that the intermediate portion of the lower case 21 does not need to begin to curve radially inward at a junction with the joint portion 23. That is, the intermediate portion of the lower case 21 may be arranged to begin to curve radially inward after extending axially downward by a predetermined distance from the joint portion 23. Also note that the intermediate portion of the lower case 21 does not need to extend radially inward in a curve to be joined to the upper end portion of the motor accommodating portion 22. That is, the intermediate portion of the lower case 21 may be arranged to first extend axially downward by a predetermined distance from the joint portion 23, and then bend radially inward substantially at a right angle to be joined to the upper end portion of the motor accommodating portion 22. Note that the predetermined distance is appropriately set within a range of the axial distance between the joint portion 23 and a lower end of the motor accommodating portion 22. Also note that the intermediate portion of lower case 21 may be arranged to obliquely extend radially inward from the joint portion 23 to be joined to the upper end portion of the motor accommodating portion 22. Also note that the intermediate portion of the lower case 21 may be arranged to obliquely extend radially inward from the joint portion 23 while changing a slant angle to be joined to the upper end portion of the motor accommodating portion 22. At least, the intermediate portion of the lower case 21 is joined to the motor accommodating portion 22 while defining a space at an axial height lower than an axial height of the joint portion 23.

An interior of the case 19 preferably includes a first annular space (an air channel portion) 50 positioned radially outward of the motor 30. Rotation of the impeller 10 causes the gas drawn in through the air inlet 40 to whirl in a circumferential direction while passing through the first annular space 50, which is arranged around the impeller 10, and to be discharged through the air outlet (not shown).

FIG. 2 is a partial cross-sectional view illustrating the first annular space 50 and its vicinity within the centrifugal fan 100 illustrated in FIG. 1 in an enlarged form.

Referring to FIG. 2, the first annular space 50 preferably includes a second annular space 51, which is a portion of a space defined by an annularly continuous collection of imaginary circles each of whose diameter D is the shortest distance between an inner circumferential edge 23 a of the joint portion 23 and a radially outermost circumferential portion 22 a of the motor accommodating portion 22 and each of which touches the inner circumferential edge 23 a of the joint portion 23, the portion of the space being defined by a portion of an inner wall of the upper case 20 and a portion of an inner wall of the lower case 21. A portion of the impeller 10 is arranged in the second annular space 51. Each imaginary circle mentioned above refers to an imaginary circle (or shape substantially arranged in the form of a circle) positioned in a plane including the rotation axis J (i.e., a plane extending in parallel or substantially in parallel with the rotation axis J).

Note that, in the structure according to the present preferred embodiment, preferably no portion of the radially outermost circumferential portion 22 a of the motor accommodating portion 22 is positioned radially inside the inner circumferential edge 23 a of the joint portion 23 at the same axial height as the axial height of the inner circumferential edge 23 a of the joint portion 23. In this case, the shortest distance between the inner circumferential edge 23 a of the joint portion 23 and the radially outermost circumferential portion 22 a of the motor accommodating portion 22 is assumed to be the shortest distance between the inner circumferential edge 23 a of the joint portion 23 and an axial extension 22 b of the radially outermost circumferential portion 22 a of the motor accommodating portion 22.

Also note that, in the present preferred embodiment, the inner circumferential edge 23 a of the joint portion 23, at which the upper and lower cases 20 and 21 are fixed to each other, is a line determined for reasons of necessity to define the second annular space 51. The position of the inner circumferential edge 23 a of the joint portion 23 does not need to be determined exactly as long as the inner circumferential edge 23 a is positioned axially below the impeller 10 and helps to define the second annular space 51, in which a whirl occurs.

In the present preferred embodiment, the second annular space 51 is arranged to define an annular air channel portion which is a portion of the first annular space 50 positioned radially outward of the motor 30, and which includes a space axially below the impeller 10. Accordingly, a circumferential whirl velocity component caused by the rotation of the impeller 10 is added to a gas which has flowed into the second annular space 51. Moreover, the gas drawn in through the air inlet 40 is incessantly sent into the second annular space 51 through the rotation of the impeller 10. The density of the gas in the second annular space 51 is consequently increased. This leads to an increase in ventilation resistance in an air channel from the air inlet 40 to the air outlet (not shown), leading to an increase in static pressure of the centrifugal fan 100.

Furthermore, in the present preferred embodiment, a portion of the impeller 10 is preferably arranged in the second annular space 51. Accordingly, the circumferential whirl velocity component is added to the gas which has flowed into the second annular space 51 through the rotation of the impeller 10. As a result, the density of the gas in the second annular space 51 is further increased. Moreover, the portion of the impeller 10 which is arranged in the second annular space 51 closes a portion of a channel arranged to contain the gas in the second annular space 51, and this leads to an increase in the ventilation resistance. As a result, the static pressure is increased. Furthermore, arranging the portion of the impeller 10 in the second annular space 51 results in a large combined surface area of portions of the impeller 10 which are positioned in the second annular space 51. As a result, the air volume of the centrifugal fan 100 is also increased.

In short, the centrifugal fan 100 according to the present preferred embodiment is able to achieve increased static pressure and an improved air volume characteristic by arranging the second annular space 51, which is annular in shape and which includes the space axially below the impeller 10, in the first annular space 50 positioned radially outward of the motor 30, and arranging a portion of the impeller 10 in the second annular space 51.

The gas drawn in through the air inlet 40 through the rotation of the first blades 12 flows into the case 19 and is guided to the first annular space 50 and then to the second annular space 51. In the present preferred embodiment, the peripheral portion of the hub 11 is arranged to project radially outward relative to the peripheral portion of each first blade 12. This makes it less likely for a gas which has been guided into the second annular space 51 to flow backward toward the first blades 12. This leads to an increase in the density of the gas in the second annular space 51 and, because the gas is pushed in the circumferential direction, to an increase in the static pressure and the air volume.

The second blades 13 are preferably arranged radially inward in the second annular space 51. The gas drawn in through the air inlet 40 flows into a radially outer space in the second annular space 51, and tends to easily stay in a radially inner space in the second annular space 51. Therefore, arranging the second blades 13 radially inward in the second annular space 51 enables a gas staying in the radially inner space in the second annular space 51 to be efficiently discharged radially outward.

Note that the second annular space 51 according to the present preferred embodiment may have any of a variety of spatial shapes. For example, a portion of the inner wall of the case 19 which covers the second annular space 51 may be arranged to include a curved surface in a vertical section. This arrangement enables the gas drawn in through the air inlet 40 to smoothly flow into the second annular space 51. Note that this curved surface is not necessarily required to be a spherical surface.

FIG. 3 is a partial cross-sectional view illustrating an example spatial shape of the second annular space 51. The first annular space 50 further includes a space 51 a positioned axially below the second annular space 51, which is the portion of the space defined by the annularly continuous collection of the imaginary circles each of whose diameter D is the shortest distance between the inner circumferential edge 23 a of the joint portion 23 and the radially outermost circumferential portion 22 a of the motor accommodating portion 22 and each of which touches the inner circumferential edge 23 a of the joint portion 23, the portion of the space being defined by a portion of the inner wall of the upper case 20 and a portion of the inner wall of the lower case 21. A gas which has been drawn in along the inner wall of the upper case 20 is guided from a radially outer portion to a radially inner portion of the space 51 a along the inner wall of the lower case 21, and stays therein as a whirl. A position at which the gas drawn in stays as the whirl depends on the axial dimension of the intermediate portion of the lower case 21. Therefore, the position at which the gas drawn in stays as the whirl according to the present preferred embodiment is axially lower than in the preferred embodiment in which the first annular space 50 does not include the space 51 a. In this case, it is desirable to elongate each second blade 13 axially downward. For example, it is desirable to elongate each second blade 13 axially downward up to the same axial position as that of the joint portion 23 or that of an upper end of the radially outermost circumferential portion 22 a of the motor accommodating portion 22 or even further downward. This causes each second blade 13 to overlap with a space in which the whirl occurs, and this enables a gas which stays as the whirl to be efficiently pushed radially outward. Note, however, that the position at which the gas drawn in stays as the whirl depends not only on the axial dimension of the intermediate portion of the lower case 21, but also on the shape of an inner wall of the intermediate portion of the lower case 21. Therefore, in addition to elongating each second blade 13 axially downward, the shape of the inner wall of the intermediate portion of the lower case 21 may be modified appropriately so that each second blade 13 will overlap with the space in which the whirl occurs.

In the present preferred embodiment, no particular limitation is imposed on the structure of the impeller 10. For example, the impeller 10 may be modified to have a structure as illustrated in FIG. 4, 5, or 6 in place of the structure illustrated in FIG. 1.

In an impeller 10 having the structure illustrated in FIG. 4, the peripheral portion of the hub 11 does not project radially outward relative to the peripheral portion of each first blade 12, but is arranged to be flush with both the peripheral portion of each first blade 12 and a peripheral portion of each second blade 13. A reduction in the radial dimension of the impeller 10 can thus be achieved.

In an impeller 10 having the structure illustrated in FIG. 5, the peripheral portion of each first blade 12 and the peripheral portion of a corresponding one of the second blades 13 are joined to each other on a radially outer side of the peripheral portion of the hub 11. The first and second blades 12 and 13 can thus be integrally defined with each other.

In an impeller 10 having the structure illustrated in FIG. 6, second blades 13 a are defined in an outer circumferential edge of the hub 11. FIG. 7 is a plan view of the impeller 10 illustrated in FIG. 6. Each second blade 13 a is preferably defined by cutting a portion of the outer circumferential edge of the hub 11 radially inward. Each second blade 13 a is preferably defined in an outer circumferential side surface of the hub 11. The outer circumferential edge of the hub 11 is locally cut away up to a vicinity of the peripheral portion of each first blade 12. In other words, a portion of the outer circumferential edge of the hub 11 begins at the vicinity of the peripheral portion of each first blade 12 and extends to a position radially outward of an adjacent one of the first blades 12. The axial extent of the hub 11 is thus able to serve as the second blades 13 a at the outer circumferential edge of the hub 11.

The structure of the hub 11 illustrated in FIG. 7 will now be described in more detail below. Each first blade 12 is arranged to extend radially outward in a smooth curve from a radially inner end thereof. In a plan view of the hub 11, a portion of the outer circumferential edge of the hub 11 begins at an outer circumferential end portion of each first blade 12 and extends radially outward along this curve up to a radially outermost point of the outer circumferential edge of the hub 11 (resulting in a first side surface), and then extends in a smooth curve from the radially outermost point of the outer circumferential edge of the hub 11 to a radially innermost point of the outer circumferential edge of the hub 11 at which an outer circumferential end portion of an adjacent one of the first blades 12 is positioned (resulting in a second side surface). The impeller 10 illustrated in FIG. 7 is arranged to rotate in a counterclockwise direction. The number of cuts defined in the outer circumferential edge of the hub 11 is preferably equal to the number of first blades 12. Note, however, that each cut may be defined for two or more of the first blades 12 (for example, for two of the first blades 12). It is enough that surfaces arranged radially should be defined in the outer circumferential edge of the hub 11 to define the second blades 13 a.

In the impeller 10 having the structure illustrated in FIG. 6, portions of the outer circumferential edge of the hub 11 are cut away as described above. Accordingly, a reduction in the weight of the impeller 10 can thus be achieved. The reduction in the weight of the impeller 10 leads to quicker start of the rotation of the impeller 10 when the impeller 10 is activated.

Each second blade 13 a of the impeller 10 having the structure illustrated in FIG. 6 is arranged in the second annular space 51. This contributes to discharging a gas which stays in the annular air channel portion efficiently radially outward.

FIG. 8 is a schematic cross-sectional view illustrating the structure of a centrifugal fan 110 according to another preferred embodiment of the present invention. The centrifugal fan 110 according to the present preferred embodiment is preferably a so-called mixed flow fan.

The structure of an impeller 10 according to the present preferred embodiment is different from the structure of the impeller 10 illustrated in FIG. 1. The structure of a motor 30 according to the present preferred embodiment is preferably identical or substantially identical to the structure of the motor 30 illustrated in FIG. 1, and a description thereof is therefore omitted.

The impeller 10 according to the present preferred embodiment is joined to a shaft 31 arranged to rotate about a rotation axis J, and includes a hub 11 arranged to obliquely extend axially downward and radially outward from the shaft 31, and a plurality of first blades 12 arranged axially above the hub 11. A portion 12 a of each first blade 12 is arranged in a second annular space 51. Note that the definition of the second annular space 51 according to the present preferred embodiment is the same as the definition of the second annular space 51 illustrated in FIG. 2.

In the centrifugal fan 110 according to the present preferred embodiment, each first blade 12 is arranged to obliquely extend axially downward and radially outward away from the rotation axis J. A gas drawn in through an air inlet 40 is accordingly sent axially downward and radially outward through rotation of the first blades 12. This enables the gas drawn in through the air inlet 40 to be smoothly guided into the second annular space 51.

In addition, an annular shroud 14 may be arranged on an axially upper end portion of each first blade 12. Here, the shroud 14 is positioned axially over the second annular space 51. This makes it less likely for the gas guided into the second annular space 51 from the air inlet 40 through the rotation of the first blades 12 to flow backward toward upper portions of the first blades 12. This leads to an increase in the density of the gas in the second annular space 51 and, because the gas is pushed in the circumferential direction, also to an increase in the static pressure.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims. 

What is claimed is:
 1. A centrifugal fan comprising: an impeller arranged to rotate about a rotation axis; a motor arranged to rotate the impeller; and a case arranged to accommodate the impeller and the motor; wherein the case includes: an upper case; a lower case including a motor accommodating portion arranged to cover an outer circumference of the motor; and a joint portion at which the upper and lower cases are fixed to each other; an interior of the case includes a first annular space positioned radially outward of the motor; the first annular space includes a second annular space defined by a portion of a space defined by an annularly continuous collection of imaginary circles each of whose diameter is a shortest distance between an inner circumferential edge of the joint portion and a radially outermost circumferential portion of the motor accommodating portion and each of which touches the inner circumferential edge of the joint portion, the portion of the space being defined by a portion of an inner wall of the upper case and a portion of an inner wall of the lower case; and a portion of the impeller is arranged in the second annular space.
 2. The centrifugal fan according to claim 1, wherein the first annular space further includes a space positioned axially below the second annular space.
 3. The centrifugal fan according to claim 1, wherein the impeller includes: a hub having a substantially circular plate shape, and joined to a shaft arranged to rotate about the rotation axis; a plurality of first blades arranged axially above the hub; and a plurality of second blades arranged axially below the hub; and the second blades are arranged in the second annular space.
 4. The centrifugal fan according to claim 1, wherein the impeller includes: a hub having a substantially circular plate shape, and joined to a shaft arranged to rotate about the rotation axis; a plurality of first blades arranged axially above the hub; and a plurality of second blades each of which is defined by a cut away portion of an outer circumferential edge of the hub radially inward; and the second blades are arranged in the second annular space.
 5. The centrifugal fan according to claim 3, wherein a peripheral portion of the hub is arranged to project radially outward relative to a peripheral portion of each first blade.
 6. The centrifugal fan according to claim 3, wherein each first blade is arranged to extend axially downward and radially outward away from the rotation axis; and a portion of an inner wall of the case which covers each first blade includes an inner wall surface arranged to extend along an axially upper end portion of each first blade.
 7. The centrifugal fan according to claim 1, wherein a portion of an inner wall of the case which covers the second annular space includes a curved surface in a vertical section.
 8. The centrifugal fan according to claim 3, wherein the second blades are arranged radially inward within the second annular space.
 9. The centrifugal fan according to claim 1, wherein the impeller includes: a hub joined to a shaft arranged to rotate about the rotation axis, and arranged to obliquely extend axially downward and radially outward from the shaft; and a plurality of first blades arranged axially above the hub; and a portion of each first blade is arranged in the second annular space.
 10. The centrifugal fan according to claim 9, further comprising an annular shroud arranged on an axially upper end portion of each first blade, wherein the shroud is positioned axially over the second annular space. 